This invention relates in one aspect to a method of loading a self-expanding stent into a delivery sheath, in which the stent in a radially confined delivery configuration is advanced axially into the sheath for delivery to a stenting site in which the sheath is withdrawn to release the stent for radial expansion. In another aspect, the invention relates to a self-expanding stent within a percutaneous transluminal delivery catheter that includes a sheath that withdraws proximally to release the stent at a stenting site, and a pusher within the sheath that retains the stent at the site during withdrawal of the sheath.
EP-A-788 332 discloses a self-expanding braided metallic stent tube and a delivery system that includes a soft annulus within the stent lumen that deforms and mechanically engages with the mesh of the stent for restraining the stent from axial movement relative to the inner catheter of the delivery system, during axial movement of a sleeve surrounding the stent. The disclosure of EP-A-596 145 is similar.
EP-A-836 447 discloses a system for delivering a self-expanding stent, in which a stopper ring on an inner catheter abuts the proximal end of the stent tube during proximal withdrawal of a sheath which surrounds the stent.
The number of materials that are biologically compatible, and available for making stents, are comparatively few. One preferred material is stainless steel. One can make stainless steel stents that are plastically deformed when they are expanded radially at the stenting site. One convenient way to expand such stents is by a balloon at the distal end of a balloon catheter. Otherwise, one can design a stainless steel stent to expand elastically when released at a stenting site. Typically, this is achieved by proximal withdrawal of a sheath on the distal end of the delivery catheter, that withdraws proximally to release the stent progressively, starting at its distal end.
Another suitable material is the nickel titanium shape memory alloy known under the trade mark NITINOL. Such stents are typically loaded into a delivery system at a low temperature when the crystal structure of the material is martensitic, and with a memory of a radially expanded shape, characteristic of a higher temperature austenitic crystalline structure. Remarkably, the nickel titanium material is biologically compatible and the martensite/austenite transformation occurs between room temperature and body temperature.
This invention is particularly applicable to self-expanding stents, irrespective of the mechanism of resilient radial expansion at the stenting site. However, the present Applicant has particular experience with nickel titanium shape memory alloy stents and the particular embodiments described below are based on such materials.
The tubular envelope of a stent usually has apertures through its wall thickness to permit radial expansion. Thus, an uncovered or “bare” stent has a tube wall that is normally liquid-permeable. However, there are many occasions when a stent with a liquid-impermeable wall that is not apertured would be desirable. To meet these needs, a family of “covered” stents have been developed. Applicant has particular experience with stent tubes provided with a covering of expanded polytetrafluoroethylene (ePTFE). Typically, the stent tube is covered by luminal and abluminal covering layers of ePTFE, which are bonded to each other through the apertures in the stent tube wall.
During manufacture of stents and delivery systems, attention must be paid to sterility. Specifically, one needs procedures for loading a covered stent into a catheter delivery system that will allow sterile conditions to be maintained, or at least thereafter achieved.
Typically, to introduce a covered self-expanding stent into a catheter delivery system, a tool needs to be provided that compresses the covered stent radially inwardly, down to a diameter which is smaller than the available diameter of the lumen of the delivery system that is to receive the compressed covered stent. Clearly, any structure within the lumen of the stent that resists further inward compression is better avoided, when the objective is to compress the stent radially inwardly as much as the system will tolerate, so as to keep the outside diameter of the delivery system at its distal tip as small as possible.
However, the stent has to be maintained at the stenting site during proximal withdrawal of the surrounding sheath, for progressive release of the stent at the stenting site. If there is no structure within the lumen of the stent, then the entire stress imposed on the stent, to prevent it moving proximally with the proximally withdrawing surrounding sheath, has to be carried on the proximal end annulus of the compressed stent. Often this is not really a problem, especially when the stent is short and not particularly highly compressed radially inwardly, and especially when friction between the compressed stent and the surrounding sheath can be brought to a particularly low value.
Nevertheless, it is important for management of fatigue resistance to avoid imposing on any point of the stent tube a level of stress that is higher than the designed maximum. A stent tube made of metal is susceptible to fatigue failure, if only because it is subject to cyclic stress at the frequency of the heartbeat of the body in which it is installed. For this reason, regulatory authorities require stringent fatigue performance standards which impose on manufacturers of stents and delivery systems an onerous burden to avoid any unforeseen stresses on the stent tube.
The state of the art contains numerous suggestions to use an element within the lumen of the stent to restrain the stent from proximal withdrawal when the surrounding sleeve is withdrawn proximally. However, these systems are of interest only for bare stents, because they rely upon mechanical interaction between surfaces on the stent pusher within the stent lumen, and boundary surfaces of apertures within the wall thickness of the stent tube.